The present invention relates to a temperature compensated oscillator in which temperature characteristics of a crystal oscillator using a crystal resonator are compensated.
A crystal oscillator using a crystal resonator is superior in stability of frequency to other oscillators, but when it is used as a reference oscillator for mobile radio communication in recent years, variations in oscillation frequency caused by temperature characteristics of the crystal resonator present a problem. In order to solve the problem, a so-called temperature compensated oscillator is widely used in which temperature characteristics of a crystal resonator are compensated.
Among the temperature compensated oscillators, one by a method called indirect method has been reduced in the number of parts and improved in performance with recent developments in the integrated circuit technology.
The principle of compensating temperature of the temperature compensated oscillator by the indirect method is explained using FIG. 17.
A temperature detection circuit 91 in FIG. 17 generates a temperature detection voltage depending on a temperature. The voltage is inputted to a high temperature part/low temperature part separation circuit 92 and a gradient correction voltage generation circuit 93. The high temperature part/low temperature part separation circuit 92 separates the voltage inputted thereto into two for a low temperature part and for a high temperature part and inputs them to a low temperature part cubic curve voltage generation circuit 94 and a high temperature part cubic curve voltage generation circuit 95, respectively.
Voltages individually outputted from the low temperature part cubic curve voltage generation circuit 94, the high temperature part cubic curve voltage generation circuit 95, the gradient correction voltage generation circuit 93, and a standard frequency adjustment voltage generation circuit 96 are inputted to an adding circuit 97 to be added, and outputted to a frequency adjustment circuit 98.
The frequency adjustment circuit 98 controls an oscillation frequency of an oscillation circuit 99, having a crystal resonator 90 by the inputted voltage. Further, the frequency adjustment circuit 98 adjusts a standard oscillation frequency at a prescribed temperature by the voltage outputted from the standard frequency adjustment voltage generation circuit 96.
The cubic curve voltage generation circuit only generates a voltage obtained by cubing the inputted voltage, and thus it can only generate a voltage in a first quadrant which is half of a cubic curve in a two-dimensional plane of the input voltage and the output voltage.
Hence, in order to obtain sequential cubic curve voltages, it is necessary to use the low temperature part cubic curve voltage generation circuit 94 which generates a cubic curve voltage by inverting the input voltage and the output voltage and the high temperature part cubic curve voltage generation circuit 95 which generates a cubic curve voltage by an normal operation, and to add the respective output voltages.
This requires the high temperature part/low temperature part separation circuit 92, the low temperature part cubic curve voltage generation circuit 94, and the high temperature part cubic curve voltage generation circuit 95.
In the above-described series of operations, the low temperature part cubic curve voltage generation circuit 94 and the high temperature part cubic curve voltage generation circuit 95 generate voltages such that the frequency adjustment circuit 98 compensates cubic temperature characteristics of an AT cut crystal, and the gradient correction voltage generation circuit 93 generates a voltage such that the frequency adjustment circuit 98 compensates linear temperature characteristics of the AT cut crystal.
These voltages are added in the adding circuit 97 and inputted to the frequency adjustment circuit 98, so as to compensate the oscillation frequency of the oscillation circuit 99 changing due to temperature. In such a manner, the oscillation frequency of the temperature compensated oscillator can be held constant, even if the temperature changes.
Such a conventional technique, however, has problems that since the high temperature part/low temperature part separation circuit for separating the voltage from the temperature detection circuit for a low temperature part and a high temperature part, the two cubic curve voltage generation circuits, the gradient correction voltage generation circuit, and the adding circuit are required in order to compensate the temperature characteristics of the AT cut crystal as described above, the circuit increases in size, and that the above circuits individually require complicated adjustment in order to correct variations in fabrication.
Hence, it is an object of the present invention to solve the problems and to provide a temperature compensated oscillator that has a simple circuit configuration suitable for downsizing and requires just easy adjustment.
In order to attain the above object, a temperature compensated oscillator according to the invention, which comprises: an oscillation circuit; a frequency adjustment circuit for changing an oscillation frequency of the oscillation circuit by a control voltage; a temperature detection circuit for detecting a temperature in the vicinity of the oscillation circuit and generating at least one output voltage based on the detected temperature; and a control voltage generation circuit including a cubic term voltage generation circuit for generating a cubic term voltage as the control voltage based on the output voltage from the temperature detection circuit, is characterized in that the cubic term voltage generation circuit is configured as follows:
Specifically, the cubic term voltage generation circuit comprises: a first MOS transistor having a source connected to a first power line; a second MOS transistor having a conduction type different from that of the first MOS transistor and a source connected to a second power line; and a first gate voltage generation circuit for generating a first gate voltage and a second gate voltage generation circuit for generating a second gate voltage respectively based on the output voltage of the temperature detection circuit.
Further, an output terminal for outputting the first gate voltage of the first gate voltage generation circuit is connected to a gate of the first MOS transistor, an output terminal for outputting the second gate voltage of the second gate voltage generating circuit is connected to a gate of the second MOS transistor, and a drain of the first MOS transistor and a drain of the second MOS transistor are commonly connected to be an output terminal of the control voltage.
It is preferable that the second power line has a polarity opposite to that of the first power line or is at the ground potential.
Further, in the case of a temperature compensated circuit in which the control voltage generation circuit includes, in place of the cubic term voltage generation circuit, a quadratic term voltage generation circuit for generating a quadratic term voltage as the control voltage based on the output voltage from the temperature detection circuit, only the following points of the configuration of the cubic term voltage generation circuit should be changed to obtain a configuration of the quadratic term voltage generation circuit.
Specifically, the second MOS transistor has the same conduction type as that of the first MOS transistor and a source is connected to the second power line.
The second power line in this case preferably has the same polarity as that of the first power line or may be the same as the first power line.
In these temperature compensated oscillators, the output terminal of the control voltage is preferably connected to at least one arbitrary voltage source via a resistance element having a resistance value of 100 kilohms or more.
In the case of the temperature compensated oscillator which comprises the control voltage generation circuit including the cubic term voltage generation circuit, it is preferable that the output terminal of the control voltage is connected to the first power line or a power line having the same polarity as that of the first power line via a first resistance element as well as to the second power line or a power line having the same polarity as that of the second power line via a second resistance element.
It is possible to use resistance elements that are different in temperature coefficient with respect to resistance value as the first resistance element and the second resistance element.
Further, it is preferable that a plurality of pairs of resistance elements having different combinations of temperature coefficients with respect to resistance values are provided as the first resistance element and the second resistance element, and switches for selectively switching to any of the plurality of pairs of resistance elements for use are provided.
In any of the temperature compensated oscillators, it is preferable that at least one of the first and second gate voltage generation circuits is a circuit for generating the first or the second gate voltage based on a difference between the output voltage of the temperature detection circuit and an arbitrary reference voltage.
Alternatively, it is possible that at least one of the first and second gate voltage generation circuits is a circuit capable of controlling the generated gate voltage thereof based on external data. Further, it is also possible that a memory circuit for storing the external data is provided, and at least one of the first and second gate voltage generation circuits is a circuit capable of controlling the generated gate voltage thereof based on the data stored in the memory circuit.
It is adoptable that at least one of the first and second gate voltage generation circuits is a voltage division circuit for dividing a voltage difference between the output voltage of the temperature detection circuit and the arbitrary reference voltage. The arbitrary reference voltage may also be a voltage of the first power line or the second power line.
In the case of the temperature compensated oscillator which comprises the control voltage generation circuit including the cubic term voltage generation circuit, it is preferable that the control voltage generation circuit outputs the cubic term voltage generated by the cubic term voltage generation circuit as a first control voltage, further comprises a linear term voltage generation circuit for generating a linear term voltage based on the output voltage of the temperature detection circuit, and outputs the linear term voltage generated by the linear term voltage generation circuit as a second control voltage, and that the frequency adjustment circuit is a circuit for controlling the oscillation frequency of the oscillation circuit by the first control voltage and the second control voltage.
The linear term voltage generation circuit may be an operational amplifier circuit. Further, it is preferable that a memory circuit is provided which stores data from the outside and controls an amplification factor and an offset input voltage of the operational amplifier circuit based on the stored digital data.
Alternatively, the temperature detection circuit may also be a circuit which comprises two temperature sensors different in temperature gradient and divides a difference between output voltages of the two temperature sensors into an arbitrary ratio to output it as a temperature detection voltage.
It is preferable that the frequency adjustment circuit comprises a voltage variable capacitance element such as a MIS variable capacitor or the like which constitutes a load capacitance of the oscillation circuit and of which capacitance value is changed by the control voltage. In this case, it is preferable that the first control voltage is applied to one electrode of the voltage variable capacitance element and the second control voltage is applied to another electrode thereof.
Alternatively, it is also possible that the voltage variable capacitance elements are constituted by a first voltage variable capacitance element to which the first control voltage is applied and a second voltage variable capacitance element to which the second control voltage is applied which are connected in parallel.
Further, it is preferable that each source of the first and second MOS transistors in the control voltage generation circuit is connected to the first or second power line via a resistance element for limiting a drain current.
It is also adoptable that the resistance element is a digital control variable resistance circuit, and that a memory circuit is provided which is capable of controlling a resistance value of the digital control variable resistance circuit based on digital data stored therein. It is preferable that the memory circuit is capable of controlling storage and read of digital data from the outside via a serial input/output line.
It is possible to use temperature detection circuits having various types of temperature sensors and circuit configurations such as one in which temperature and the output voltage are in a proportional relationship, one in which they are in an inversely proportional relationship, one in which a plurality of temperature gradients can be selected, and the like as the above-described temperature detection circuit.
Further, when a preset temperature range in a temperature range in use of each of the temperature compensated oscillators is defined as a second temperature area, a temperature range on a lower temperature side than that is defined as a first temperature area, and a temperature range on a high temperature side exceeding the second temperature area is defined as a third temperature area, it is desirable to have a configuration as follows:
The first gate voltage generation circuit has an area in which the first gate voltage linearly changes with respect to changes in temperature at least in the third temperature area, and the second gate voltage generation circuit has an area in which the second gate voltage linearly changes with respect to changes in temperature at least in the first temperature area.